5.0.0 - Crash with prefer-const rule where code contains array destructuring assignment with 'ignored values'

[mjmasn]

This is a minimal reproduction for a bug I've found in eslint@5.0.0. As far as I can tell the example code below is valid JavaScript (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#Ignoring_some_returned_values).

Tell us about your environment

• ESLint Version: 5.0.0
• Node Version: 8.11.0
• npm Version: 5.6.0
• yarn Version: 1.7.0

What parser (default, Babel-ESLint, etc.) are you using? default

Please show your full configuration:

{
    "parserOptions": {
        "ecmaVersion": 2018
    },
    "rules": {
      "prefer-const": 1
    }
}

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

const b = [1, 2];
let a;

[, a] = b;

cd eslint-5-array-destructuring-assignment-bug
yarn eslint .

What did you expect to happen?
eslint would lint my project

What actually happened? Please include the actual, raw output from ESLint.
eslint did not lint my project and threw a TypeError from one of its built-in rules (prefer-const)

Cannot read property 'name' of null
TypeError: Cannot read property 'name' of null
    at elements.map.element (/node_modules/eslint/lib/rules/prefer-const.js:173:49)
    at Array.map (<anonymous>)
    at getIdentifierIfShouldBeConst (/node_modules/eslint/lib/rules/prefer-const.js:173:26)
    at groupByDestructuring (/node_modules/eslint/lib/rules/prefer-const.js:227:28)
    at Program:exit (/node_modules/eslint/lib/rules/prefer-const.js:362:17)
    at listeners.(anonymous function).forEach.listener (/node_modules/eslint/lib/util/safe-emitter.js:45:58)
    at Array.forEach (<anonymous>)
    at Object.emit (/node_modules/eslint/lib/util/safe-emitter.js:45:38)
    at NodeEventGenerator.applySelector (/node_modules/eslint/lib/util/node-event-generator.js:251:26)
    at NodeEventGenerator.applySelectors (/node_modules/eslint/lib/util/node-event-generator.js:280:22)

[platinumazure]

Hi @mjmasn, thanks for the issue!

I can reproduce in the demo. I'll mark this as a bug.

[mjmasn]

👍 BTW changing

eslint/lib/rules/prefer-const.js

Line 173 in 196c102

.map(element => element.name)

to

.map(element => element ? element.name : null)

fixes the crash but I'm not sure if there are any wider implications...

[platinumazure]

Thanks @mjmasn, that makes sense. Would you like to create a PR (maybe with a new test that would trigger the crash)?

[platinumazure]

Just out of curiosity, anyone know if this can be recreated in the 4.x release line?

[mjmasn]

@platinumazure I don't think so, the code was introduced in 240c1a4 (first released in 5.0.0-alpha.4)

[aladdin-add]

should we release a patch 5.0.2, since seems it was introduced in 5.0.0-alpha.4? cc @eslint/eslint-team